A driving force of an automobile is governed by an output of an engine with respect to an opening degree of an accelerator and a speed ratio of a transmission connected with the engine. Basically, such relation is determined according to a grade of a vehicle at a designing or planning stage. However, a required driving force differs according to a road condition, a driving preference of a driver of the vehicle and so on. Therefore, it is preferable to adjust characteristics of the driving force and the speed ratio in accordance with the road condition and the driving preference of the driver.
For example, PCT international publication WO2011/021634 discloses a vehicle control system configured to control characteristics of driving force, speed ratio, suspension etc. based on a command index representing driving preference of the driver that is obtained based on a synthesized acceleration of the longitudinal acceleration and lateral acceleration. In turn, Japanese Patent Laid-Open No. 2005-113946 discloses a control device for continuously variable transmission configured to change a speed ratio of the continuously variable transmission stepwise. According to the teachings of Japanese Patent Laid-Open No. 2005-113946, an upper limit speed of an engine speed is determined in view of durability of the continuously variable transmission, and a lower limit speed of the engine speed is determined in view of avoiding an engine stall. When the engine speed reaches the upper limit, an upshifting is carried out to lower the engine speed. By contrast, when the engine speed reaches the lower limit, a downshifting is carried out to raise the engine speed.
A belt driven continuously variable transmission and a toroidal type continuously variable transmissions are adapted to change a speed ratio continuously. Also, a speed ratio of a hybrid vehicle can be changed substantially continuously by controlling the engine speed arbitrarily by a motor. Therefore, a speed ratio of the vehicle having the continuously variable transmission or the hybrid vehicle is basically controlled in a manner to drive the engine at an operating point at which fuel economy is optimized. To this end, specifically, an engine speed at which the fuel economy is optimized is determined on a constant output curve governed by an engine torque and an engine speed, and a speed ratio is controlled in a manner to adjust the engine speed to the determined speed at which the fuel economy is optimized with respect to a current vehicle speed or an output speed. At the same time, an engine torque is adjusted by controlling an opening degree of a throttle valve. The invention taught by PCT international publication WO2011/021634 is applied to the vehicle thus structured to adjust the driving force in line with the driving preference of the driver, and to adjust a suspension in a manner to achieve a sporty running.
When an accelerator pedal is depressed deeply by the driver to accelerate the vehicle, a target output is changed according to an opening degree of the accelerator. However, an output of the engine is delayed inevitably to achieve the target output. Therefore, if the driver has a desire to accelerate the vehicle abruptly, the driver may further depress the accelerator pedal to cover shortage of the engine output caused by the delay in augmentation of the engine output. That is, if the driver has a desire to accelerate the vehicle greatly and promptly, the driver may depress the accelerator pedal more than necessary. As a result, the engine speed to achieve the target output will be increased to reach the upper limit speed of the vehicle. If the accelerator pedal is returned after the engine speed reached the upper limit speed so that the vehicle speed was increased, the engine speed will be lowered to achieve the required power thus reduced while optimizing the fuel economy. Thus, if the conventional vehicle in which a speed ratio is changed continuously is accelerated significantly, the engine speed may reach the maximum allowable speed of the vehicle, and when the accelerating operation is cancelled, the engine speed will be lowered significantly to the speed to optimize the fuel economy. In short, an amount of change of the engine speed is large. In addition, an intermediate speed between those speeds may not be set, that is, the engine may not be allowed to be driven at a driving point to achieve the intermediate speed. Further, when the accelerating operation is carried out to accelerate the vehicle greatly, the engine speed may reach the upper limit speed irrespective of an operating amount. Therefore, the driver may feel a delay in augmentation of the vehicle speed behind an augmentation of the engine speed.